Treasure hunters claim to have found the sunken remains of a British steamer torpedoed during the Second World War filled with platinum, now worth $3 billion (£1.9bn).

The Sub Sea Research, a company based in the northeastern US state of Maine, found the British ship SS Port Nicholson on the ocean floor around 30 miles off Provincetown, Massachusetts, the Boston Globe reported.

Greg Brooks of Sub Sea Research claims a US Treasury Department ledger shows platinum bars were on board, as part of a payment from the Soviet Union to the US for war supplies.

He also says he has video footage showing the platinum bars, although none of the bounty has been raised to the surface yet.

"I'm going to get it, one way or another, even if I have to lift the ship out of the water," he said.

"There's a good possibility there are about 10 tons of gold down there, too, and maybe some industrial diamonds," he added.

However, the claim should be viewed with scepticism, said Robert F. Marx, an underwater archaeologist, maritime historian and owner of Seven Seas Search and Salvage LLC in Florida.

Both an American company and an English company previously went after the contents of the ship years ago and surely retrieved at least a portion, Marx said. The question is how much, if any, platinum is left, he said.

"Every wreck that is lost is the richest wreck lost. Every wreck ever found is the biggest ever found. Every recovery is the biggest ever recovery," Marx said. Maritime law would also complicate ownership claims.

The Port Nicholson was sailing from Halifax in Canada to New York when it was torpedoed and sank in 1942. Four people died when the ship went down and 87 were rescued.

The Sub Sea Research team discovered the sunken treasure in August 2008 with help from a remote control machine tethered to their ship, the Sea Hunter, the paper said.

The cargo, valued in 1942 at about $53 million, was a lend-lease payment to the United States from the Soviet Union.

The treasure hunters said that there are at least 30 boxes scattered around the shipwreck.